The interior volume of the heart can be measured or estimated in a number of ways. Radiographic dilution techniques and impedance plethysmography are two widely used techniques. Impedance measures have been used to extrude volume and have also been used as a feedback parameter for implantable pacemakers and for direct hemodynamic measurements in a diagnostic setting.
Traditionally impedance measurements have been made using catheters which employ multiple ring electrodes on the distal end of a catheter. Such methods use a widely spaced pair of current driving electrodes on an elongate cylindrical catheter shaft. Typically one or more pairs of ring electrodes for sensing the resultant voltages are also placed on the shaft. This catheter is placed in a heart chamber. The measured impedance has been found to be directly related to the total volume of the chamber in which the catheter is placed.
Kuhn (IEEE Transactions on Biomedical Engineering, Vol. 40 No. 6 June 1993 pp 589-592) determined the effects of catheter movement on measurements of volume based on impedance measurements. Using such calculations, Kuhn created a look-up table which enabled him to calculate chamber volume without regard to the position of the catheter.
Prior art volume estimation techniques do not measure provide a intracavitary distances which is a shortcoming of the prior art.